1—Field of the Invention
The invention relates to a device for rotationally coupling two shafts sliding along their common axis. The coupling device of the invention applies in particular to an automobile vehicle steering column, in which it is fitted to the intermediate column part connected to the steering box or the upper column part connected to the steering wheel.
2—Description of the Prior Art
In the more particular case of the intermediate shaft, the latter has a universal joint at each end: one universal joint is connected to the movement input of the steering box and the other universal joint is connected to the column top. For correct steering of modern automobile vehicles, it is necessary for the length of the intermediate shaft to be variable and to adapt to oscillations of the front drive train of the vehicle, which oscillations are caused by the profile and the surface state of the road covering. This characteristic is also required to facilitate assembly to the pinion of the rack and pinion and to absorb movement in the event of a head-on impact to the vehicle.
The intermediate shaft must therefore have a variable length, i.e. it must have a function of sliding of two shafts relative to each other along their common axis, which is the axis of the intermediate part. Moreover, there must be a function of transmission of rotation movement between the two shafts and of transmission of the rotation torque necessary for the steering maneuver.
There are many sliding shaft coupling devices that associate the passage of the rotation torque between the two shafts by the use of splines with conjugate profiles formed on each of the two shafts. However, this type of device exhibits play after an endurance test cycle equivalent to the service life of a vehicle, which is that required on modern automobiles. To delay the onset of this play, the sliding fit of the two shafts at the fabrication stage is relatively tight, which calls for a relatively high axial force during assembly on the production line, which increases the fitting time and makes fitting more difficult.
This fit must be accurate to enable axial movements to be absorbed correctly in operation. The axial force during transmission of rotation torque is a function of the torque to be transmitted, and there is an increase in the sliding force linked to the torque to be transmitted and to the coefficient of friction, followed by a sudden axial unlocking, which causes jerks that compromise maintaining a good sliding fit with reduced play, leading to a poor driving “feel”.
There also exist coupling devices that inject plastics material over splined portions of a metal male shaft and a metal female tube. This solution gives rise to problems absorbing axial movements under high torque; the sliding forces increase in proportion to the friction forces between the two parts. Moreover, plays appear following wear of the injected plastics material. A final existing solution introduces rolling elements and stress springs between the shaft and the tube. This solution, which is satisfactory for sliding under torque, causes angular stiffness problems as it is directly proportional to the stiffness and to the prestressing of the springs.